In recent years, an idling-stop of vehicles has become an important means as one of the CO2 reduction measures in a battle against global warming.
The idling-stop is a system that stops fuel injection to an engine in order to stop the engine automatically when the vehicles stop at a crossing by a traffic light, traffic congestion, etc., for example.
In a conventional starter that starts the engine, an engine ring gear has to be stopped or a pinion cannot be engaged with the ring gear fundamentally.
For this reason, even when the idling-stop is performed because of a red light at the crossing and the light changes to a green light before a rotation of the engine stops completely, for example, the engine cannot be re-started immediately.
Therefore, it is desired that a technology that can re-start the engine immediately when a user performs a starting operation (releasing a brake, shifting into a drive range, etc., for example), that is, when a re-starting demand occurs even when the rotation of the engine is decreasing by engine inertia.
In order to realize such a technology, the pinion of the starter is needed to be engaged with the rotating ring gear.
There is a method of independently controlling an operation that pushes out the pinion to the engine ring gear side and an operation that opens and closes an electric point of contact for intermitting a current that flows into a motor, as one means to make the technology possible (refer to Japanese Patent Application Laid-Open Publication No. 2009-191843).
Important requirements in controlling independently the operation that pushes out the pinion to the ring gear side and opening-and-closing operation of the electric point of contact are relative number of rotations of the ring gear and the pinion, and it is needless to mention that the smaller the relative number of rotations, the smoother the engagement of the pinion and the ring gear.
Especially when the re-starting demand arises in a high range of the engine rotation, it becomes possible to obtain the required relative number of rotations (for example, 100 rpm or less) that the ring gear and the pinion can engage smoothly in shorter time by starting the motor before the operation that pushes out the pinion to the ring gear side, thus shortening the re-starting time, and there is an advantage also in the improvement in fuel consumption.
In addition, it is natural to consider the number of rotations of the pinion with a value converted into a gear ratio relative to the ring gear.
As mentioned above, it is necessary to acquire the rotation speed of the both ring gear and the pinion in the control that pushes out the pinion and make it engaged with the ring gear after starting the motor.
Although the rotation of the ring gear can be detected by using the rotation signal from an existing crank angle sensor etc., for example, it is necessary to detect the rotation of the pinion using an independent rotation detector.
For example, as shown in Japanese Patent Application Laid-Open Publication No. 2009-529114, a method of detecting the rotation of the pinion by the rotation detector disposed near the pinion, or a method that an armature shaft of the motor is extended in an axial direction to outside of an end frame, detecting the rotation of the armature shaft by the rotation detector disposed near the extended end of the shaft, and obtain the rotation speed of the pinion from the rotation of the armature shaft, or the like can be considered.
However, when using the independent rotation detector in order to detect the rotation of the pinion, when detecting the rotation of the pinion directly, or when the detecting rotation of the armature shaft extended outside of the frame and obtaining the rotation speed of the pinion, the rotation detector should be disposed outside the starter.
Since a mounting space for the rotation detector is needed in these cases and mountability is restricted, it is difficult to arrange a rotation detector at a suitable place.
On the other hand, Japanese Patent Application Laid-Open Publication No. 2011-29138 discloses a controlling method that is provided with no rotation detector to the pinion, but a rotation starting characteristic of the motor is memorized in the internal memory of a control device, and the rotating speed of the pinion estimated based on the lapsed time after turning on the motor.
However, since the motor used for the starter is generally a direct-current commutator motor, there is a defect that a rotation speed is not stabilized easily with time.
That is, since mechanical loss decreases by wear of a brush and an electric loss increases, the rotation behavior changes easily.
It is likely that when the rotation speed changes, the suitable relative number of rotations of the pinion to the ring gear also changes, which increases the noise at the time of engagement, and in a worst case, wear of the gears advances.
Therefore, since it is necessary to compose a control logic that predicts the change of the rotation behavior in the conventional technology disclosed in '138, control becomes complicated, and it is difficult to perform precise control in order to synchronize the rotation of the pinion with the ring gear whose rotation is slowing.